1. Field of the Invention
The presently claimed invention relates to radiation hardened optical fibers and a method of making the same. More particularly, the presently claimed invention relates to silica core fibers (i.e. SiO.sub.2 core fiber) with or without an F doped silica cladding wherein such fibers are hardened to reduce radiation induced losses in the visible wavelength range (i.e. 400-700 nm) and a method of making the same.
2. Description of the Related Art
It is recognized that pure synthetic silica core fibers exhibit relatively large permanent radiation induced losses to transmissions below about 650 nm. See West, R. H. and Lenham, A. P., Radiation-Hardened Pure Silica-Core Fibre Optics, Electronic Letters, Vol. 19 (16), pp. 622-623 (Aug. 4, 1983), incorporated herein by reference in its entirety and for all purposes. West et al. comment that "in this lower wavelength region . . . the hardening exposures themselves produce a relatively large permanent loss, typically 1000 dB/km after 10.sup.4 Gy." Id at 622. The B-2 fibers (i.e. fibers containing 750 ppm of OH, 100 ppm of Cl in the core and containing a cladding of F-doped silica) of Nagasawa et al., infra, and the OH-800-B fibers (i.e. fibers containing 700 ppm of OH, 100 ppm of Cl in the core and containing a silicone resin cladding) also of Nagasawa et al., infra, exhibit absorption peaks in the wavelength region from 600 to 630 nm after .gamma.-ray irradiation of 12 kGy (i.e. 1.2.times.10.sup.4 Gray=1.2.times.10.sup.6 rad). The magnitude of the absorption peaks are 2000 dB/km and 250 dB/km in the B-2 and OH-800-B fibers, respectively. See Nagasawa et al., Radiation Effects on Pure Silica Core Optical Fibers by .gamma.-Rays: Relation between 2 eV Band and Non-Bridging Oxygen Hole Centers, Japanese Journal of Applied Physics, Vol. 25 (3), pp. 464-468 (March, 1986), incorporated herein by reference in its entirety and for all purposes. See also Nagasawa et al. at pp. 465, FIG. 2. Due to absorption bands appearing in the 400-700 nm visible spectrum range, pure synthetic silica (PSS) core fibers have not been useful for transmitting images in the visible range wherein such optical fibers are exposed to varying levels of .gamma.-irradiation.
If pure synthetic silica fibers could be manufactured that have the ability to transmit images in the 400-700 nm wavelength range during extended exposure to .gamma.-irradiation, such fibers would be useful in various applications. Such applications include optical fibers for monitoring tokamak fusion reactors, image guides for visual inspection of fission reactors, image guides for visual inspection of nuclear waste repositories and image guides amenable to sterilizing .gamma.-irradiation (i.e. without loss of transmission in the visible wavelength range) prior to use in medical instrumentation of various types.
The shortcomings of irradiation induced transmission losses in the visible wavelength range (i.e. about 400-700 nm) of pure synthetic silica fibers is fully documented. See Nagasawa, K., Tanabe, M., and Yahagi, K., Gamma-Ray Induced Absorption Bands in Pure-Silica-Core Fibers, Japanese Journal of Applied Physics, Vol. 23 (12), pp. 1608-1613 (December, 1984), incorporated herein by reference in its entirety and for all purposes. See Friebele et al., Radiation-Induced Optical Absorption Bands in Low Loss Optical Fiber Waveguides, Journal of Non-Crystalline Solids 38 & 39, pp. 245-250 (1980), incorporated herein by reference in its entirety and for all purposes. See Nagasawa et al., Gamma-Ray Induced Absorption Band at 770 nm in Pure Silica Core Optical Fibers, Japanese Journal of Applied Physics, Vol. 23 (5), pp. 606-611 (May, 1984), incorporated herein by reference in its entirety and for all purposes. FIG. 3 of Nagasawa et al. in the Japanese Journal of Applied Physics, Vol. 23 (5) at p. 608 illustrates that upon exposure of various pure synthetic silica fibers to doses of gamma-irradiation on the order of 1.2.times.10.sup.4 Gray at a dose rate of 250 Gray/hour, induced losses on the order of about 600 to 800 dB/km appear in the visible wavelength region. See also Sigel et al., Radiation Response of Large Core Polymer Clad Silica Optical Fibers, IEEE Transactions on Nuclear Science, Vol. NS-26 (6), pp. 4796-4801 (Dec. 6, 1979), incorporated herein by reference in its entirety and for all purposes. See also Nagasawa et al., Effect of Cladding Material on 2-eV Optical Absorption in Pure-Silica Core Fibers and Method to Suppress the Absorption, Japanese Journal of Applied Physics, Vol. 26 (1) pp. 148-151 (January, 1987) wherein the authors state that "when optical fibers are used to guide a visual image, it is essential to suppress the optical absorption in the visible-wavelength region . . . . The absorption band at around 630 nm, which is sometimes called the 2-eV absorption band, needs to be suppressed." Id at 148. Nagasawa et al. suggest that "the 2-eV absorption can be reduced by a combined-treatment of hydrogen and irradiation . . . . The authors believe that this treatment is valid to improve radiation resistance of any fiber." Id at 150. While the suggested irradiation and hydrogen may be useful to reduce gamma irradiation induced losses in the visible wavelength region, it is further recognized that such treatment is only partially successful in reducing such losses. Various difficulties including trapping sufficient amounts of hydrogen, having to hermetically seal fibers against outdiffusion of hydrogen and undue expense are encountered. See also D. L. Griscom, Defect Structure of Glasses, Journal of Non-Crystalline Solids Vol. 73, pp. 51-77 (1985), incorporated herein by reference in its entirety and for all purposes.
Radiation hardening is a process wherein a permanent reduction of induced loss occurs during continuous irradiation. See West, R. H., A Local View of Radiation Effects in Fiber Optics, Journal of Lightwave Technology, Vol. 6 (2), pp. 155-164 (1988), incorporated herein by reference in its entirety and for all purposes. While radiation hardening has been observed in the IR wavelength range, radiation hardening sufficient to transmit visible spectral images has to date not been successfully accomplished wherein induced losses are below about 30 dB/km in pure synthetic silica fibers at doses of at least 100,000 Gray.